The environmental conditions which integrated circuits must endure during operation are severe, particularly in an automotive environment. During the operation of an automobile, the integrated circuit chips are exposed to large variations in temperature arising from not only the changes in ambient atmospheric temperature, but also from the variations in the operating temperature generated by the automobile itself. Generally, an automobile manufacturer requires these integrated circuits to reliably perform while experiencing repeated cycling between ambient temperatures of -40.degree. C. and +125.degree. C. Therefore, it is necessary that the integrated circuit be capable of withstanding these temperature extremes without detriment to its structural or electrical integrity.
In addition, power generating integrated circuits are particularly problematic in that they accordingly also, generate substantial amounts of heat during generation of the power. This heat generation increases the temperature of the chip and may detrimentally affect circuit performance and/or cause chip failure. In addition, the power generating integrated circuit may need to be dielectrically isolated from the surrounding components. These detrimental effects are compounded when combined with the temperature extremes experienced by the integrated circuit during operation of an automobile. Therefore, it is desirable to provide means for maintaining a constant temperature of the integrated circuit chip despite any variations in temperature within and surrounding the chip. A common solution involves soldering a chip to a heat sink. Generally the heat sinks are formed from aluminum, which is relatively inexpensive, or alternatively copper, which is relatively expensive.
However, a shortcoming of this approach is that during the exposure to the variations in temperature, the solder used to couple the integrated circuit chip and heat sink will fracture due to stresses arising from the differences in thermal expansion rates of the various materials. The coefficient of thermal expansion for silicon is approximately equal to 3.5.times.10.sup.-6 /.degree. C., while the coefficients of thermal expansion for copper and aluminum are approximately equal to 17.7.times.10.sup.-6 /.degree. C. and 24.times.10.sup.-6 /.degree. C., respectively. Generally, the solder will fracture most often at the corners of the chip where the stresses are concentrated These fractures are particularly undesirable as they increase the thermal resistance between the chip and the heat sink, which in turn causes the temperature of the chip to rise because of retarded heat transfer to the heat sink. This phenomenon further adversely affects the performance of the integrated circuit thereby promoting failure of the integrated circuit chip
Alternative solutions to the temperature problem have also been disclosed by the prior art and generally involve the use of an interface device, or buffer, between the integrated circuit chip and heat sink which is characterized by a high thermal conductivity, or correspondingly, relatively low thermal resistance. In addition, it is desirable that the interface device have a coefficient of thermal expansion which is intermediate between that of the integrated circuit chip and that of the heat sink. In such a structure, the integrated circuit is soldered to one side of the interface device and the heat sink to the other side of the interface device Molybdenum and tungsten have been commonly used for interface devices because of their appropriate heat transfer and expansion characteristics. These interface devices have been a satisfactory solution to the problem, however it is desirable to provide a less expensive alternative In particular, for automotive applications, molybdenum and tungsten are considered too expensive for commercial use.
Another type of interface device has been proposed in United States patent application Ser. No. 191,441, entitled "Integrated Circuit Heat Sink Interface Apparatus" to Akin et al, wherein an inner core of low expansion material is surrounded everywhere by a copper outer layer characterized by high thermal conductivity. The integrated circuit contacts the copper layer at a region which is over the inner core layer of low expansion material. The heat sink contacts the copper layer on an opposite side of the integrated circuit. This arrangement is of relatively low cost and permits rapid conductivity of the generated heat from the integrated circuit through the copper outer layer around the inner core layer, while the low expansion material retards the overall thermal expansion of the interface device. Although this design is satisfactory, it would be desirable to provide an interface device which permits the most efficient heat transfer directly from the integrated circuit to the heat sink, rather than around the inner core layer as in this previous method.
Therefore, it is desirable to provide an interface device which provides direct thermal coupling between an integrated circuit and heat sink, and which is durable, reliable over varying temperature conditions, characterized by an intermediate coefficient of expansion, and readily amenable to automotive production techniques.